The present invention relates to a method of manufacturing a micromachine such as an optical switching element used for communication, measurement, display, and the like which utilize switching.
There is a MEMS element which is fabricated by a micromachine technique of performing three-dimensional micropatterning by etching based on thin film formation or photolithography. Of MEMS elements serving as micromachines, one is comprised of a fine fixed structure and movable structure, and controls the operation of the movable structure by an electrical signal. Such MEMS element is an optical switching element whose movable structure has a reflecting surface (reference 1: Japanese Patent Laid-Open No. 2001-198897, reference 2: Japanese Patent Laid-Open No. 2002-189178, reference 3: Japanese Patent Laid-Open No. 11-119123, reference 4: xe2x80x9cMEMS Micro Technology, Mega Impactxe2x80x9d Circuit and Device, pp. 14-25 (2001), reference 5: Renshi Sawada, Eiji Higurashi, Akira Shimizu, and Tohru Maruno, xe2x80x9cSingle Crystalline Mirror Actuated Electrostatically by Terraced Electrodes With High-Aspect Ratio Torsion Spring,xe2x80x9d Optical MEMS 2001, pp. 23-24 (Okinawa Japan), 2001, and reference 6: Renshi Sawada, Johji Yamaguchi, Eiji Higurashi, Akira Shimizu, Tsuyoshi Yamamoto, Nobuyuki Takeuchi, and Yuji Uenishi, xe2x80x9cSingle Si Crystal 1024ch MEMS Mirror Based on Terraced Electrodes and a High-Aspect Ratio Torsion Spring for 3-D Cross-Connect Switch,xe2x80x9d Optical MEMS 2002, pp. 11-12 (Lugano Switzerland), 2002).
The optical switching device is comprised of, e.g., a fixed structure and movable reflecting structure. The reflecting structure has a support member and movable member, and the movable member is coupled to the support member by a spring member such as a torsion spring. The optical switch with this arrangement performs switching operation of switching the optical path by moving the reflecting structure by the attractive force or repulsive force between the fixed structure and the movable reflecting structure.
As an optical switching element manufacturing method, a method using an SOI (Silicon On Insulator) substrate is proposed. A process of fabricating a mirror (movable portion) by this method will be explained. As shown in FIG. 4A, grooves 401a are formed by known photolithography and etching such as DEEP RIE on a side (major surface: SOI layer) of an SOI substrate 401 on which a buried oxide 402 is formed, thereby forming a mirror 404 from a single-crystal silicon layer 403 on the buried oxide 402.
In DEEP RIE, for example, SF6 gas and C4F8 gas are alternately introduced in dry-etching silicon. Etching and formation of a side wall protective film are repeated to form a groove or hole with an aspect ratio of 50 at an etching rate of several xcexcm/min.
A resist pattern which is open In the formation region of the mirror 404 is formed on the lower surface of the SOI substrate 401. Silicon is selectively etched from the lower surface of the SOI substrate 401 by using an etching solution such as an aqueous solution of potassium hydroxide. In etching, the buried oxide 402 is used as an etching stopper layer. As shown in FIG. 4B, an opening 401b is formed at a portion of the lower surface of the SOI substrate 401 that corresponds to the formation region of the mirror 404. The opening 401b is a region corresponding to the pixel of the optical switching element.
The region where the buried oxide 402 is exposed through the opening 401b is selectively removed with hydrofluoric acid, forming the pivotal mirror 404 supported by the SOI substrate 401, as shown in FIG. 4C. To increase the reflectance of the mirror 404, a metal film of gold or the like may be formed on the surface of the mirror 404 on the opening 401b side.
A silicon substrate 411 is selectively etched with an aqueous solution of potassium hydroxide by using as a mask a predetermined mask pattern formed from a silicon nitride film or silicon oxide film, thus forming a recessed structure, as shown in FIG. 4D. A metal film is formed on the recessed structure by vapor deposition or the like. The metal film is patterned by photolithography and etching using known ultra-deep exposure, thereby forming an electrode 412 including a mirror driving electrode interconnection and the like, as shown in FIG. 4E.
After that, the SOI substrate 401 and silicon substrate 411 are diced into chips, thus forming a mirror chip and electrode chip. The mirror chip and electrode chip are adhered into an optical switching element in which the mirror 404 can be moved by applying an electric field, as shown in FIG. 4F. After each chip is diced, a metal film of gold or the like may be formed on the mirror surface in order to increase the mirror reflectance.
In a step after etching a buried oxide according to the conventional manufacturing method, the mirror portion is coupled by a pair of coupling members so as to be pivotal on a pivot shaft which extends through the coupling members. The coupling members are bar- or plate-like spring members such as torsion bar springs which elastically deform upon application of torsion.
For example, while being coupled by torsion bar springs, the mirror undergoes a wafer dry step after etching a buried oxide with a buffered hydrofluoric acid solution and cleaning the buried oxide with water, a wafer dicing step, a step of forming a metal film on a diced mirror surface, a step of adhering a mirror chip to a substrate bearing a mirror driving electrode interconnection, a step of bonding a die to a package, a wire bonding step, a potting step, and the like.
The optical switching element applies an attractive force to the mirror by an electric field generated by a voltage applied to the mirror driving electrode, and pivots the mirror through an angle of several degrees. For reduction in power consumption and the like, the mirror must be pivoted by applying a voltage of about 100 V to the mirror driving electrode. Thus, the coupling member is processed into a width of about 2 xcexcm so as to easily pivot the mirror.
Since the SOI layer is about 10 xcexcm thick, the coupling member is about 2 xcexcm wide and 10 xcexcm thick. For example, as shown in FIG. 5, a circular mirror 501 having a diameter of about 500 xcexcm is coupled to a surrounding concentric mirror frame 502 via thin coupling members 511 having a width of about 2 xcexcm. The mirror frame 502 is coupled to an SOI layer 503 via coupling members 512.
In the above-mentioned steps, a water flow, a centrifugal force in drying a wafer, vibrations, or shocks are applied. This readily damages a coupling member or mirror, decreasing the manufacturing yield of the mirror substrate. Especially when even one mirror becomes defective on a mirror substrate on which many mirrors are arrayed in a matrix, the mirror substrate becomes a defective and cannot be used, resulting in a lower yield.
When a mirror substrate wafer is transported as a wafer or diced chip after the manufacture, the wafer or chip itself is protected by a vessel which stores it. However, a mirror and mirror frame which are coupled by thin coupling members are movable and vulnerable to the centrifugal force, vibrations, and shocks. The manufacturing yield of the mirror substrate may further decrease.
The manufacture of an optical switching element mirror substrate is completed when a mirror surface which reflects incident light is exposed. In dicing into a chip, small wafer shaving powder is attached to the mirror surface via the gap of the coupling member or the like. Dust is attached in safekeeping or handling till packaging, decreasing the optical reflectance.
It is a principal object of the present invention to manufacture a micromachine having a movable portion such as a mirror at a high nondefective yield.
To achieve the above object, according to one aspect of the present invention, there is provided a micromachine manufacturing method comprising at least the step of preparing a silicon substrate having a single-crystal silicon layer on an upper surface via a buried oxide, the movable portion formation step of selectively etching the single-crystal silicon layer by using a movable portion formation mask pattern as a mask, thereby forming on the single-crystal silicon layer a movable portion which is coupled to the surrounding single-crystal silicon layer via a coupling portion on the buried oxide, the frame formation step of selectively etching away the silicon substrate from a lower surface by using as a mask a frame formation mask pattern having an opening, thereby forming a substrate opening in the silicon substrate and exposing a lower surface of the buried oxide in the substrate opening, the movable portion protective film formation step of forming a movable portion protective film on the single-crystal silicon layer so as to cover the movable portion while the movable portion is formed on the buried oxide, the buried oxide processing step of forming in a movable portion formation region of the buried oxide a movable portion opening which communicates with the substrate opening while the movable portion protective film is formed, and the step of forming a buried protective film which covers the movable portion exposed in the substrate opening and the movable portion opening, and the single-crystal silicon layer around the movable portion while the movable portion protective film is formed.
With this arrangement, the movable portion formed in the single-crystal silicon layer is kept fixed by bringing some layer or film into contact with the movable portion until the buried protective film is formed.